Active thin film transistor (TFT) LCDs are becoming increasingly popular due to the generally superior image quality which these displays can provide in comparison to, for example, passive displays. A typical TFT LCD includes a plurality of TFT LCD elements which include a liquid crystal element including a pair of electrodes which sandwich a portion of liquid crystal material, typically a twisted nematic (TN) liquid crystal material. A voltage applied across the electrodes by a TFT integrated with the liquid crystal element can be used to modulate the amount of light transmitted through the liquid crystal material.
As illustrated in FIG. 1, a TFT LCD element typically includes a thin-film transistor TFT which has a first controlled electrode connected to a data line Dn and a second controlled electrode connected to an electrode of a liquid crystal element, here shown as a liquid crystal capacitance Clc connected between the thin film transistor TFT and a common electrode Vcom. A voltage typically is applied across the liquid crystal capacitance Clc by driving the gate of the thin-film capacitor TFT to turn on the transistor TFT and applying a voltage from the data line Dn to the liquid crystal element Clc. The voltage, i.e., the data, is maintained across the liquid crystal element Clc after the transistor TFT is turned off due to the capacitance of the element Clc and a storage capacitor Cst connected to the liquid crystal element Clc. In a conventional LCD, the storage capacitors of a particular row of LCD elements typically are connected to the gate line which drives the TFTs of an adjacent row of LCD elements. However, for the first row of LCD elements driven by a first gate line G1, the storage capacitors Cst are connected to a dummy gate line G0 which typically is not used to drive thin-film transistors.
As illustrated in FIG. 2, the common electrodes Vcom are typically driven by a voltage having a periodic waveform. During a time Ton when the transistor TFT is driven "on" by the gate line G1, a voltage Vp is applied to the liquid crystal element Clc, causing a voltage Vlc to be established across the liquid crystal element Clc, which is maintained after the transistor TFT is turned "off." The dummy gate line typically is driven by a periodic voltage Voff, resulting in a voltage Vst across the storage capacitor Cst. The voltage VG0 used to drive the dummy gate line G0 typically is the same as the "off" portion of the gate driving voltage VG1 used to drive the gate of the transistor TFT.
Unfortunately, this method of driving the first row of LCD elements may cause nonuniform performance for the LCD. Because the impedance of the dummy gate line G0 may differ from the impedance of the regular gate line G1 due to the lack of the additional capacitance provided by the gates of the thin-film transistors TFT, the first row of LCD elements may perform differently than the other rows of LCD elements in the LCD. Reduced capacitance on the dummy gate line may allow the liquid crystal elements to more quickly discharge. Thus, if normally "white" mode LCD elements are employed in the LCD, i.e., elements which become transparent when less voltage is applied across the liquid crystal element Clc, the first row of LCD elements may appear brighter than the other rows of the display.